Cathodic electrocoat composition

ABSTRACT

An electrocoat coating composition comprising an electrodepositable resin that has functionality reactive with isocyanate and a curing agent having at least one allophanate groups is described. The invention also provides a method of applying the composition of the invention to a substrate and curing the applied coating.

FIELD OF THE INVENTION

The invention concerns thermosetting electrocoat primer compositionsthat have curing agents based on polyisocyanates.

BACKGROUND OF THE INVENTION

Electrodeposition coating compositions and methods are widely used inindustry today. One of the advantages of electrocoat compositions andprocesses is that the applied coating composition forms a uniform andcontiguous layer over a variety of metallic substrates regardless ofshape or configuration. This is especially advantageous when the coatingis applied as an anticorrosive coating onto a substrate having anirregular surface, such as a motor vehicle body. The even, continuouscoating layer over all portions of the metallic substrate providesmaximum anticorrosion effectiveness.

Electrocoat baths usually comprise an aqueous dispersion of a principalfilm-forming resin, such as an acrylic or epoxy resin, having ionicstabilization. For automotive or industrial applications for which hardelectrocoat films are desired, the electrocoat compositions areformulated to be curable compositions. This is usually accomplished byincluding in the bath a crosslinking agent that can react withfunctional groups on the principal resin under appropriate conditions(such as with the application of heat) and thus cure the coating. Duringelectrodeposition, coating material containing an ionically-chargedresin having a relatively low molecular weight is deposited onto aconductive substrate by submerging the substrate in an electrocoat bathhaving dispersed therein the charged resin and then applying anelectrical potential between the substrate and a pole of oppositecharge, for example, a stainless steel electrode. The charged coatingmaterial migrates to and deposits on the conductive substrate. Thecoated substrate is then heated to cure the coating.

One curing mechanism utilizes a melamine formaldehyde resin curing agentin the electrodepositable coating composition to react with hydroxylfunctional groups on the electrodeposited resin. This curing methodprovides good cure at relatively low temperatures (perhaps 130° C.), butthe crosslink bonds contain undesirable ether linkages and the resultingcoatings provide poor overall corrosion resistance as well as poor chipand cyclic corrosion resistance.

In order to address some of the problems with melamine cross-linkedelectrocoats, many commercial compositions employ polyisocyanatecrosslinkers to react with hydroxyl or amine functional groups on theelectrodeposited resin. This curing method provides desirable urethaneor urea crosslink bonds, but it also entails several disadvantages. Inorder to prevent premature gelation of the electrodepositable coatingcompositions, the highly reactive isocyanate groups on the curing agentmust be blocked. Blocked polyisocyanates, however, require hightemperatures, typically 175° C. or more to unblock and begin the curingreaction. In the past, the isocyanate crosslinkers have been blockedwith a compound such as an oxime or alcohol, which unblocks andvolatilizes during cure, in order to provide the lowest temperatures forthe unblocking and curing reactions. The volatile blocking agentsreleased during cure can cause other deleterious effects on variouscoating properties, however, and increase organic emissions. There isthus a need for electrodepositable coating compositions that couldprovide desirable urethane or urea crosslink linkages but that avoid theproblems that now accompany compositions having polyisocyanate curingagents blocked with volatilizing agents.

SUMMARY OF THE INVENTION

We have now invented electrocoat coating compositions that have loweremissions upon curing of the coating and that typically cure at lowertemperatures than current blocked-isocyanate electrocoat compositions.The compositions of the present invention comprise a curing agent havingat least one allophanate group. The electrocoat coating compositions ofthe invention also have unexpectedly improved throwpower properties.

The present invention further provides a method of coating a conductivesubstrate. In the method of the invention, a conductive substrate isimmersed in an electrodeposition coating composition comprising, in anaqueous medium, an ionic resin and a curing agent having at least oneallophanate group. A potential of electric current is applied betweenthe conductive substrate and an electrode of the opposite charge todeposit a coating layer onto the conductive substrate. The depositedcoating is cured by reaction between the ionic resin and the curingagent having at least one allophanate group.

DETAILED DESCRIPTION OF THE INVENTION

The electrocoat compositions of the present invention comprise, in anaqueous medium, an ionic principal resin and a curing agent having atleast one allophanate group. The allophanate crosslinkers of theinvention may be prepared by a method that includes a first step ofpreparing an allophanate compound that has isocyanate functionality andan optional second step of reacting at least some of the residualisocyanate groups with a blocking agent and/or an isocyanate-reactiveextender compound to produce a blocked isocyanate and/or higherfunctionality crosslinker.

In the first step, the allophanate group or groups of the allophanatecompound are formed by reacting an excess of equivalents of organicpolyisocyanate with a mono- or polyhydric compound in the presence of acatalyst. The reaction is understood to involve formation of an initialurethane group which then, in the presence of an appropriate catalyst,further reacts to form allophanate. The amount of mono- or polyhydriccompound employed should usually not exceed one-half equivalent of mono-or polyhydric compound per equivalent of isocyanate to avoid productshaving an unusably high viscosity. In general, there should be about0.01 to about 0.5 equivalents of hydroxyl per equivalent of isocyanate.A more preferred range would be from about 0.1 to about 0.3 equivalentsof hydroxyl per equivalent of isocyanate. Although reaction conditionsmay be varied, typically the reaction may continue for 3 to 10 hours attemperatures of perhaps about 60° C. to about 150° C. Progress of thereaction can be monitored by any of the usual methods, such astitration, infrared spectroscopy, or viscosity measurement. A catalystdeactivator may optionally be added to stop the allophanate formation ata point where the desired isocyanate content or viscosity has beenobtained. Addition of a deactivator is also desirable for storagestability of the product with unreacted isocyanate content.

Organic polyisocyanates that may be employed to prepare the allophanatecontaining compound include aromatic, aliphatic, and cycloaliphaticpolyisocyanates and combinations thereof. Representative of usefulpolyisocyanates are diisocyanates such as m-phenylene diisocyanate,2,4-toluene diisocyanate, 2,6-toluene diisocyanate, mixtures of 2,4- and2,6-toluene diisocyanate, hexamethylene diisocyanate, tetramethylenediisocyanate, cyclohexane-1,4-diisocyanate, any of the isomers ofhexahydrotoluene diisocyanate, isophorone diisocyanate, any of theisomers of hydrogenated diphenylmethane diisocyanate,naphthalene-1,5-diisocyanate, 1-methoxyphenyl-2,4-diisocyanate, any ofthe isomers of diphenylmethane diisocyanate, including2,2′-diphenylmethane diisocyanate, 2,4′-diphenylmethane diisocyanate,and 4,4′-diphenylmethane diisocyanate, isomers of biphenylenediisocyanate including 2,2′-, 2,4′-, and 4,4′-biphenylene diisocyanates,3,3′-dimethoxy-4,4′-biphenyl diisocyanate and3,3′-dimethyl-diphenylmethane-4,4′-diisocyanate; triisocyanates such as4,4′,4″-triphenylmethane triisocyanate and toluene 2,4,6-triisocyanate;and the tetraisocyanates such as4,4′-dimethyldiphenylmethane-2,2′,5,5′-tetraisocyanate; and polymericpolyisocyanates such as polymethylene polyphenylene polyisocyanate.Especially useful due to their availability and properties are thevarious isomers of toluene diisocyanate and diphenylmethane diisocyanateand combinations of those isomers. Modified isocyanates, includingisocyanurates, biurets, uretdione, and carbodiimide modifications arealso advantageously used to produce the allophanate crosslinker.

The mono- and polyhydric compounds that may be reacted with thepolyisocyanate may have an equivalent weight of about 30 to about 1000,can contain up to about 8 hydroxyl groups in the molecule, and can alsobe alkylene oxide adducts of lower molecular weight alcohols. Monohydricalcohols that may be employed include both aliphatic and aromaticalcohols. Suitable examples include, without limitation, methanol,ethanol, propanol, 2-propanol, n-butanol, 2-chloroethanol, pentanol,n-octanol, 2-ethylhexanol, isooctyl alcohol, nonanol, ethylene glycolmonoalkyl ethers, propylene glycol monoalkyl ethers, diethylene glycolmonoalkyl ethers and higher molecular weight analogs of polyethyleneglycol monoalkyl ethers, dipropylene glycol monoalkyl ethers and highermolecular weight analogs of polypropylene glycol monoalkyl ethers,3,5,5-trimethylhexanol, isodecyl alcohol, benzyl alcohol, phenol,cyclohexanol, 2,2,2-tricholoroethanol, and the like, alkylene oxideadducts thereof, and combinations of these. The alkylene oxide may beethylene oxide, propylene oxide, butylene oxide, pentylene oxide, orcombinations thereof.

Suitable polyhydric alcohols include both aliphatic and aromaticcompounds. Particular examples include, without limitation, ethyleneglycol, diethylene glycol, and higher polyethylene glycol analogs liketriethylene glycol; propylene glycol, dipropylene glycol, and higherpolypropylene glycol analogs like tripropylene glycol; 1,4-butanediol,1,3-butanediol, 1,6-hexanediol, 1,7-heptanediol, glycerine,1,1,1-trimethylolpropane, 1,1,1-trimethylolethane, hexane-1,2,6-triol,pentaerythritol, sorbitol, 4,4′-isopropylidene diphenol, (bisphenol A),resorcinol, catechol, hydroquinone, alkylene oxide adducts thereof andcombinations of these.

Suitable catalysts for the reaction are any that are known to promoteformation of allophanate linkages. These include, without limitation,metal carboxylates, alcoholates, oxides, phenolates and metal chelates.The preferred catalysts are acetylacetonates, including zinc, cobalt,nickel, ferric, and aluminum acetylacetonates, and tin compounds,including dibutyltin dilaurate, dibutyltin oxide, stannous octoate, anddibutyltin diacetate.

The reaction may be continued until all of the isocyanate groups havereacted. In this case, when there is no residual isocyanatefunctionality after the allophanate reaction, the allophanate compoundmay be used in the electrocoat coating composition without furthermodification. In one preferred embodiment, however, the allophanatesynthesis is ended while isocyanate functionality still remains. Thereaction may be stopped with residual functionality, for example, tocontrol the viscosity of the allophanate-functional product. It ispreferred to have a viscosity, measured at 50° C., of 500,000 cps orless, more preferably 100,000 cps or less. In one particularly preferredembodiment, the allophanate reaction is continued until an isocyanateequivalent weight is obtained that is from about 200 to about 1200, morepreferably from about 250 to about 1000, and even more preferably fromabout 250 to about 400.

The reaction may effectively be stopped by reducing the temperature, butit is often preferable to add a catalyst deactivator at the desiredpoint of the reaction. Examples of the catalyst deactivators that mayoptionally be employed at the end of the reaction to prevent furtherallophanate formation include, without limitation, aliphatic andaromatic acid chlorides such as acetyl chloride, benzoyl chloride,benzenesulfonyl chloride, oxalyl chloride, adipyl chloride, sebacylchloride, carbonyl chloride, and combinations of such compounds.Inorganic acid deactivators such as perchloric acid and strong organicacids such as trifluoromethanesulfonic acid and trifluoroacetic acid mayalso be used. Another group of catalyst deactivators that may be usedare chloroformates such as methyl chloroformate, ethyl chloroformate,isopropyl chloroformate, n-butyl chloroformate, sec-butyl chloroformate,and diethylene glycol bis chloroformate.

In a preferred embodiment, the isocyanate-functional allophanatecompound may be reacted in a second step to block the residualisocyanate groups and/or to extend the compound through reaction of theresidual isocyanate groups. Suitable blocking agent are those compoundsthat will unblock under the curing conditions to regenerate theisocyanate group for reaction as a crosslinking site. Blocking agentssuitable for crosslinkers for electrocoat coating compositions are knownin the art and include, without limitation, oximes, lower alcohols,lactams, and phenol. Specific examples of such materials include,without limitation, ethylene glycol monobutyl ether, diethylene glycolmonobutyl ether, methyl ethyl ketoxime, ε-caprolactam, and phenol.

Alternatively or in addition to reaction with a blocking agent, theisocyanate-functional allophanate precursor compound may be reacted withan extender compound, which is an isocyanate reactive material that isnot expected to unblock and regenerate the isocyanate functionalityduring the curing reactions. Preferably, the extender compound is apolyfunctional compound that has two or more functional groups selectedfrom primary amine groups, secondary amine groups, and alcohol groups.The polyfunctional extender compounds act as extenders to link two ormore molecules of the allophanate precursor, producing a crosslinkerwith more allophanate groups per molecule. Useful examples of extendercompounds include aminoalcohols, polyfunctional amines, and polyols.Particular examples of such materials include, without limitation,trimethylolpropane, diethyl toluene diamine, trifunctional ordifunctional polyoxyalkylene amines (available commercially under thetradename POLYAMINE® from BASF Corporation or under the tradenameJEFFAMINE® from Huntsman). The crosslinker preferably has no residualisocyanate functionality.

The allophanate compound used as the curing agent of the invention hasat least about one allophanate group per molecule on average andpreferably has a plurality of allophanate groups per molecule. Thecuring agent preferably has up to about 16, more preferably up to about12, and even more preferably up to about 8 allophanate groups permolecule on average. The curing agent also has preferably more thanabout 1, more preferably at least about 2, and even more preferably atleast about 3 allophanate groups per molecule, on average. Theallophanate compound curing agent of the invention preferably has fromabout 1 to about 16 allophanate groups on average per molecule, morepreferably has from about 1 to about 12 allophanate groups on averageper molecule, and even more preferably has from about 1 to about 8allophanate groups on average per molecule. Typically, the crosslinkermay have an equivalent weight of from about 200 to about 1200, based oncombined equivalents of allophanate and blocked isocyanate groups (ifpresent). The weight average molecular weight may vary widely. In apreferred embodiment, the crosslinker of the invention has a weightaverage molecular weight of from about 2000 to about 15,000.

The electrocoat composition is an aqueous dispersion that includes atleast a principal film-forming resin and the allophanate curing agent ofthe invention. A variety of such resins are known, including withoutlimitation, acrylic, polyester, epoxy, and polybutadiene resinsPreferably, the principal resin is cathodic, i.e., it has basic groupsand is salted with an acid. In a cathodic electrocoating process, thearticle to be coated is the cathode. Water-dispersible resins used inthe cathodic electrodeposition coating process have a cationicfunctional group such as primary, secondary, and/or tertiary aminemoiety as a positively chargeable hydrophilic group.

In a preferred embodiment, the resin is an epoxy resin functionalizedwith amine groups. Preferably, the epoxy resin is prepared from apolyglycidyl ether. Preferably, the polyglycidyl ether of is thepolyglycidyl ether of bisphenol A or similar polyphenols. It may also beadvantageous to extend the epoxy resin by reacting an excess of epoxidegroup equivalents with a modifying material, such as a polyol, apolyamine or a polycarboxylic acid, in order to improve the filmproperties. Preferably, the polyglycidyl ether is extended withbisphenol A. Useful epoxy resins of this kind have a weight averagemolecular weight, which can be determined by GPC, of from about 3000 toabout 6000. Epoxy equivalent weights can range from about 200 to about2500, and are preferably from about 500 to about 1500.

Amino groups can be incorporated by reacting the polyglycidyl ethers ofthe polyphenols with amine or polyamines. Typical amines and polyaminesinclude, without limitation, dibutylamine, ethylenediamine,diethylenetriamine, triethylenetetramine, dimethylaminopropylamine,dimethylaminobutylamine, diethylaminopropylamine,diethylaminobutylamine, dipropylamine, and similar compounds, andcombinations thereof. In a preferred embodiment, the epoxide groups onthe epoxy resin are reacted with a compound comprising a secondary aminegroup and at least one latent primary amine. The latent primary aminegroup is preferably a ketimine group. After reaction with the epoxy theprimary amines are regenerated, resulting in an amine-capped epoxyresin. Resins used according to the invention preferably have a primaryamine equivalent weight of about 300 to about 3000, and more preferablyof about 850 to about 1300.

Epoxy-modified novolacs can be used as the resin in the presentinvention. The epoxy-novolac resin can be capped in the same way aspreviously described for the epoxy resin.

Acrylic polymers may be made cathodic by incorporation ofamino-containing monomers, such as acrylamide, methacrylamide, dimethylamino ethyl methacrylate or t-butyl amino ethyl methacrylate.Alternatively, epoxy groups may be incorporated by including anepoxy-functional monomer in the polymerization reaction. Suchepoxy-functional acrylic polymers may be made cathodic by reaction ofthe epoxy groups with polyamines according to the methods previouslydescribed for the epoxy resins. The molecular weight of a typicalacrylic resin is usually in the range from about 2000 to about 50,000,and preferably from about 3000 to about 15,000.

Cationic polyurethanes and polyesters may also be used. Such materialsmay be prepared by endcapping with, for example, an aminoalcohol or, inthe case of the polyurethane, the same compound comprising a saltableamine group previously described may also be useful.

Polybutadiene, polyisoprene, or other epoxy-modified rubber-basedpolymers can be used as the resin in the present invention. Theepoxy-rubber can be capped with a compound comprising a saltable aminegroup.

In an alternative embodiment, cationic or anionic acrylic resins may beused. In the case of a cationic acrylic resin, the resin is polymerizedusing N,N′-dimethylaminoethyl methacrylate, tert-butylaminoethylmethacrylate, 2-vinylpyridine, 4-vinylpyridine, vinylpyrrolidine orother such amino monomers. In the case of an anionic acrylic resin, theresin is polymerized using acrylic acid, methacrylic acid, crotonicacid, maleic acid, fumaric acid, crotonic acid, isocrotonic acid,vinylacetic acid, and itaconic acid, anhydrides of these acids, or othersuitable acid monomers or anhydride monomers that will generate an acidgroup for salting. The polymerization also includes ahydroxyl-functional monomer. Useful hydroxyl-functional ethylenicallyunsaturated monomers include, without limitation, hydroxyethylmethacrylate, hydroxyethyl acrylate, hydroxypropyl acrylate,hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutylmethacrylate, the reaction product of methacrylic acid with styreneoxide, and so on. Preferred hydroxyl monomers are methacrylic or acrylicacid esters in which the hydroxyl-bearing alcohol portion of thecompound is a linear or branched hydroxy alkyl moiety having from 1 toabout 8 carbon atoms. The monomer bearing the hydroxyl group and themonomer bearing the group for salting (amine for a cationic group oracid or anhydride for anionic group) may be polymerized with one or moreother ethylenically unsaturated monomers. Such monomers forcopolymerization are known in the art. Illustrative examples include,without limitation, alkyl esters of acrylic or methacrylic acid, e.g.,methyl methacrylate, ethyl acrylate, ethyl methacrylate, propylacrylate, propyl methacrylate, isopropyl acrylate, isopropylmethacrylate, butyl acrylate, butyl methacrylate, isobutyl acrylate,isobutyl methacrylate, t-butyl acrylate, t-butyl methacrylate, amylacrylate, amyl methacrylate, isoamyl acrylate, isoamyl methacrylate,hexyl acrylate, hexyl methacrylate, 2-ethylhexyl acrylate, decylacrylate, decyl methacrylate, isodecyl acrylate, isodecyl methacrylate,dodecyl acrylate, dodecyl methacrylate, cyclohexyl acrylate, cyclohexylmethacrylate, substituted cyclohexyl acrylates and methacrylates,3,5,5-trimethylhexyl acrylate, 3,5,5-trimethylhexyl methacrylate, thecorresponding esters of maleic, fumaric, crotonic, isocrotonic,vinylacetic, and itaconic acids, and the like; and vinyl monomers suchas styrene, t-butyl styrene, alpha-methyl styrene, vinyl toluene and thelike. Other useful polymerizable co-monomers include, for example,alkoxyethyl acrylates and methacrylates, acryloxy acrylates andmethacrylates, and compounds such as acrylonitrile, methacrylonitrile,acrolein, and methacrolein. Combinations of these are usually employed.

The amino equivalent weight of the cationic resin can range from about150 to about 5000, and preferably from about 500 to about 2000. Thehydroxyl equivalent weight of the resins, if they have hydroxyl groups,is generally between about 150 and about 2000, and preferably about 200to about 800.

The electrodeposition coating composition may further containconventional pigments such as titanium dioxide, ferric oxide, carbonblack, aluminum silicate, precipitated barium sulfate, aluminumphosphomolybdate, strontium chromate, basic lead silicate or leadchromate. The pigments may be dispersed using a grind resin or,preferably, a pigment dispersant. The pigment-to-resin weight ratio inthe electrocoat bath can be important and should be preferably less than50:100, more preferably less than 40:100, and usually about 10 to30:100. Higher pigment-to-resin solids weight ratios have been found toadversely affect coalescence and flow. Usually, the pigment is 10-40percent by weight of the nonvolatile material in the bath. Preferably,the pigment is 15 to 30 percent by weight of the nonvolatile material inthe bath. Any of the pigments and fillers generally used in electrocoatprimers may be included. Extenders such as clay and anti-corrosionpigments are commonly included.

The above components are uniformly dispersed in an aqueous medium.Usually, the principal resin and the crosslinking agent are blendedtogether before the resins are dispersed in the water. In a preferredembodiment, the amine groups of the cathodic electrocoat resins aresalted with an acid, such as phosphoric acid, propionic acid, aceticacid, lactic acid, or citric acid. The salting acid may be blended withthe resins, mixed with the water, or both, before the resins are addedto the water. The acid is used in an amount sufficient to neutralizeenough of the amine groups of the principal resin to impartwater-dispersibility to the resin. The cationic resin may be fullyneutralized; however, partial neutralization is usually sufficient toimpart the required water-dispersibility. By “partial neutralization” wemean that at least one, but less than all, of the basic groups on theresin are neutralized. By saying that the cationic resin is at leastpartially neutralized, we mean that at least one of the basic groups onthe resin is neutralized, and up to all of such groups may beneutralized. The degree of neutralization that is required to afford therequisite water-dispersibility for a particular resin will depend uponits chemical composition, molecular weight, and other such factors andcan readily be determined by one of ordinary skill in the art throughstraightforward experimentation.

Similarly, the acid groups of an anionic resin are salted with an aminesuch as dimethylethanolamine or triethylamine. Again, the salting agent(in this case, an amine) may be blended with the resins, mixed with thewater, or both, before the resins are added to the water. The anionicprincipal resin is at least partially neutralized, but may be fullyneutralized as in the case of the cationic resin. At least enough acidgroups are salted with the amine to impart water-dispersibility to theresin.

Besides water, the aqueous medium of an electrocoat composition may alsocontain a coalescing solvent. Useful coalescing solvents includehydrocarbons, alcohols, esters, ethers and ketones. The preferredcoalescing solvents include alcohols, polyols and ketones. Specificcoalescing solvents include monobutyl and monohexyl ethers of ethyleneglycol, and phenyl ether of propylene glycol, monoalkyl ethers ofethylene glycol such as the monomethyl, monoethyl, monopropyl, andmonobutyl ethers of ethylene glycol; dialkyl ethers of ethylene glycolsuch as ethylene glycol dimethyl ether; or diacetone alcohol. A smallamount of a water-immiscible organic solvent such as xylene, toluene,methyl isobutyl ketone or 2-ethylhexanol may be added to the mixture ofwater and the water-miscible organic solvent. The amount of coalescingsolvent is not critical and is generally between about 0 to 15 percentby weight, preferably about 0.5 to 5 percent by weight based on totalweight of the resin solids.

The electrodeposition coating compositions used in the invention cancontain optional ingredients such as dyes, flow control agents,plasticizers, catalysts, wetting agents, surfactants, UV absorbers, HALScompounds, antioxidants, defoamers and so forth. Examples of surfactantsand wetting agents include alkyl imidazolines such as those availablefrom Ciba-Geigy Industrial Chemicals as AMINE C® acetylenic alcoholssuch as those available from Air Products and Chemicals under thetradename SURFYNOL®. Surfactants and wetting agents, when present,typically amount to up to 2 percent by weight resin solids. Plasticizersare optionally included to promote flow or modify plating properties.Examples are high boiling water immiscible materials such as ethylene orpropylene oxide adducts of nonyl phenols or bisphenol A. Plasticizerscan be used at levels of up to 15 percent by weight resin solids.

Curing catalysts such as tin catalysts can be used in the coatingcomposition. Typical examples are without limitation, tin and bismuthcompounds including dibutyltin dilaurate, dibutyltin oxide, and bismuthoctoate. When used, catalysts are typically present in amounts of about0.05 to 2 percent by weight tin based on weight of total resin solids.

The electrocoat bath generally has an electroconductivity from 800micromhos to 6000 micromhos. When conductivity is too low, it isdifficult to obtain a film of desired thickness and having desiredproperties. On the other hand, if the composition is too conductive,problems such as the dissolution of substrate or counter electrode inthe bath, uneven film thickness, rupturing of the film, or poorresistance of the film to corrosion or water spotting may result.

The coating composition according to the present invention iselectrodeposited onto a substrate and then cured to form a coatedarticle. The electrodeposition of the coating preparations according tothe invention may be carried out by any of a number of processes knownto those skilled in the art. The electrodeposition coating compositionmay be applied on any conductive substrate, such as steel, copper,aluminum, or other metals or metal alloys, preferably to a dry filmthickness of 10 to 35 μm. The article coated with the composition of theinvention may be a metallic automotive part or body. After application,the coated article is removed from the bath and rinsed with deionizedwater. The coating may be cured under appropriate conditions, forexample by baking at from about 275° F. to about 375° F. for betweenabout 15 and about 60 minutes.

Following electrodeposition, the applied coating is usually cured beforeother coatings, if used, are applied. When the electrocoat layer is usedas a primer in automotive applications, one or more additional coatinglayers, such as a primer-surfacer, color coat, and, optionally, aclearcoat layer, may be applied over the electrocoat layer. The colorcoat may be a topcoat enamel. In the automotive industry, the color coatis often a basecoat that is overcoated with a clearcoat layer. Theprimer surfacer and the topcoat enamel or basecoat and clearcoatcomposite topcoat may be ether waterborne or solventborne. The coatingscan be formulated and applied in a number of different ways known in theart. For example, the resin used can be an acrylic, a polyurethane, or apolyester. Typical topcoat formulations are described in U.S. Pat. Nos.4,791,168, 4,414,357, 4,546,046, 5,373,069, and 5,474,811. The coatingscan be cured by any of the known mechanisms and curing agents, such as amelamine or blocked isocyanate.

The invention is further described in the following example. The exampleis merely illustrative and does not in any way limit the scope of theinvention as described and claimed. All parts are parts by weight unlessotherwise noted.

EXAMPLE 1

Preparation of Crosslinker Having Allophanate Groups

A reaction vessel equipped with an addition funnel, stirrer, andthermometer was charged with 2757.3 grams of LUPERNATE® MI (an isomericblend of diphenylmethane diisocyanate available from BASF Corp.) and2.58 grams of zinc acetylacetonate monohydrate. Stirring was begun, andthe reactor content were heated to about 60-63° C. An addition of 1038.5grams of ethylene glycol monobutyl ether was carried out at a constantrate over a period of one hour. Thereafter, the content of the flaskwere heated to about 103-106° C. and maintained at that temperature forfive hours. Then, 1.53 grams of benzoyl chloride were added and thereaction mixture was allowed to cool to ambient temperature. Theallophanate precursor product had a free NCO content of 8.05% by weightand a viscosity of 59,200 cps at 50° C. An FTIR spectrum of the productincluded absorptions attributable to allophanate linkages.

A clean, suitable reactor was charged with 522 grams of the allophanateprecursor and 130.5 grams of anhydrous methyl isobutyl ketone. Thesolution was heated to 38° C. and then 0.6 grams of dibutyl tindilaurate was added. A total of 59.0 grams of ethylene glycol monobutylether was added dropwise to the reactor over a period of about 20minutes while maintaining a temperature of 43-56° C. The reactionmixture was held at 65° C. for an hour. The measured isocyanateequivalent weight at that point was 1366. A total of 22.4 grams oftrimethylolpropane was added in portions over 15 minutes whilemaintaining a temperature between 65-75° C. The reaction mixture washeld for two hours at 75° C. until the isocyanate functionality wasconsumed. The reaction product was diluted with 7.4 grams of n-butanoland 120.5 grams of methyl isobutyl ketone. The product had a nonvolatilecontent of 72%, a viscosity of 2800 cps at 25° C., and a weight averagemolecular weight (determine by GPC using a styrene standard) of 3065.The theoretical available isocyanate equivalent weight was 545.

EXAMPLE 2

Preparation of Electrocoat Emulsion

In a suitable container, 823.6 grams of an epoxy solution (812 weightper epoxide) is held at a temperature of 115° C. for addition of 622.6grams of the crosslinker of Example 1. Then, 152.6 grams of aplasticizer mixture (59% nonvolatiles) was then added. At 85° C., 44.1grams of the diketimine of diethylene triamine, 48.9 grams ofmethylethanolamine, and 14.0 grams of propylene glycol phenyl ether wereadded. After 30 minutes, 15 grams of dimethylaminopropylamine were addedand the temperature maintained at 100° C. for 30 minutes. The mixturewas reduced to 76% nonvolatile by weight with 114.8 grams isobutanol toproduce the final resin mixture.

A two-gallon vessel was charged with 940.7 grams of deionized water and61.5 grams of 88% lactic acid. The acid number was adjusted to 37 mgKOH/gram. An amount of 1841 grams of the final resin mixture was addedwith good mixing. A total of 1902 additional grams of deionized waterwere added in portions with good mixing to produce an emulsion with anonvolatile content of 30% by weight. Organic solvent was stripped fromthe emulsion and additional deionized water added. The final emulsionhad a viscosity of 35 cps, a nonvolatile content of 28% by weight, a pHof 6.0, and a conductivity of 1800 micromhos. The extent ofneutralization was 46%.

In a separate container, 2500 grams of the final emulsion, 217.1 gramsof a pigment paste (60% by weight nonvolatile, pigment-to-binder =3.5),and 1467 grams of deionized water were mixed together. The electrocoatbath was mixed for 2 hours in an open vessel. The bath had a nonvolatilecontent of 20% by weight, and pH of 5.4, and a conductivity of 1777micromhos.

EXAMPLE 3

Production of Cured Electrocoat Coating

Using a DC rectifier, 4″×12″ steel panels were coated as the cathode ofan electrodeposition cell with the electrocoat bath at 90° F. The panelwas coated from 2.2 minutes at a set voltage was 275 volts for a curedfilmbuild of 0.63 mil.

The panels were cured at 325° F. or 350° F. to determine the percentweight loss during cure. The cured films were smooth and continuous. Theweight loss during cure was compared to that of a commercial product,CATHOGUARD® 310G, available from BASF Corp., having a standard blockedisocyanate crosslinker.

Cure Temperature Example 3 Commercial Control 325° F. 9.24% 11.57% 350°F. 11.79% 13.56%

The invention has been described in detail with reference to preferredembodiments thereof. It should be understood, however, that variationsand modifications can be made within the spirit and scope of theinvention and of the following claims.

What is claimed is:
 1. An electrocoat coating composition comprising, inan aqueous medium, (a) an ionic resin having functionality reactive withisocyanate groups and (b) a compound comprising a plurality ofallophanate groups.
 2. An electrocoat coating composition according toclaim 1, wherein the compound (b) is prepared by a process comprising astep of reacting a hydroxyl-functional compound with a polyisocyanate ina ratio of from about 0.01 to about 0.5 equivalents of alcohol perequivalent of isocyanate.
 3. An electrocoat coating compositionaccording to claim 2, wherein the ratio of alcohol to isocyanate is fromabout 0.1 to about 0.3 equivalents of alcohol per equivalent ofisocyanate.
 4. An electrocoat coating composition according to claim 2,wherein the alcohol is selected from the group consisting of n-butanol,2-chloroethanol, 2-ethylhexanol, ethylene glycol monoalkyl ethers,propylene glycol monoalkyl ethers, benzyl alcohol, phenol, ethyleneglycol polyethylene glycols, propylene glycol, polypropylene glycols,butanediols, trimethylolpropane, pentaerythritol, and alkylene oxideadducts thereof, and combinations thereof.
 5. An electrocoat coatingcomposition according to claim 1, wherein the compound (b) is thereaction product of reaction of: (b)(1) an isocyanate-functionalprecursor having a plurality of allophanate groups and (b)(2) a compoundselected from the group consisting of blocking agents, extendercompounds, and combinations thereof.
 6. An electrocoat coatingcomposition according to claim 5, wherein the compound (b)(2) comprisesa blocking agent.
 7. An electrocoat coating composition according toclaim 5, wherein the compound (b)(2) is selected from the groupconsisting of oximes, lactams, phenol, ethylene glycol monobutyl ether,diethylene glycol monobutyl ether, and combinations thereof.
 8. Anelectrocoat coating composition according to claim 5, wherein thecompound (b)(2) comprises a compound that has two or more functionalgroups selected from the group consisting of primary amine groups,secondary amine groups, alcohol groups, and combinations thereof.
 9. Anelectrocoat coating composition according to claim 8, wherein thecompound that has two or more functional groups is selected from thegroup consisting of trimethylolpropane, diethyl toluene, diamine,trifunctional polyoxyalkylene amines, difunctional polyoxyalkyleneamines, and combinations thereof.
 10. An electrocoat coating compositionaccording to claim 1, wherein the compound (b) comprises an allophanateof a polyisocyanate selected from the group consisting of isomers ofdiphenyl methane diisocyanate and mixtures thereof.
 11. An electrocoatcoating according to claim 1, wherein the compound (b) comprises anallophanate of a polyisocyanate selected from the group consisting ofisocyanurates, biurets, uretdione-containing compounds, andcarbodiimide-containing compounds.
 12. An electrocoat coatingcomposition according to claim 1, wherein the compound (b) comprisesfrom about 3 to about 16 allophanate groups on average per molecule. 13.An electrocoat coating composition according to claim 1, wherein thecompound (b) has an equivalent weight of from about 200 to about 1200,based on combined equivalents of allophanate and blocked isocyanategroups.
 14. An electrocoat coating composition according to claim 1,wherein the compound (a) is an epoxy resin comprising amine groupssalted with an acid.
 15. An electrocoat coating composition according toclaim 1, wherein the compound (a) is an acrylic resin comprising acidgroups salted with an amine.
 16. A method of coating a conductivesubstrate, comprising the steps of: (a) providing an aqueous electrocoatcoating composition comprising an ionic resin having functionalityreactive with isocyanate and a compound comprising a plurality ofallophanate groups; (b) immersing a conductive substrate in saidelectrocoat coating composition; (c) applying a potential of electriccurrent between an electrode and the conductive substrate to deposit acoating layer onto the conductive substrate; and (d) curing thedeposited coating layer by reacting of the resin having functionalityreactive with isocyanate and the compound comprising a plurality ofallophanate groups.
 17. A method according to claim 16, wherein theionic resin is a cationic epoxy resin.
 18. A method according to claim16, wherein the compound comprising a plurality of allophanate groups isthe reaction product of (a) an isocyanate-functional precursor having aplurality of allophanate groups and (b) a compound selected from thegroup consisting of blocking agents, extender compounds, andcombinations thereof.
 19. A method according to claim 18, wherein thecompound comprising a plurality of allophanate groups has an equivalentweight of from about 200 to about 1200, based on combined equivalents ofallophanate and blocked isocyanate groups.
 20. A method according toclaim 18, wherein the compound comprising a plurality of allophanategroups has from about 3 to about 16 allophanate groups.
 21. A coatedsubstrate prepared according to the method of claim
 16. 22. A method ofpreparing an electrocoat coating composition, comprising steps of: (a)combining a resin having functionality reactive with isocyanate groupsand ionizable functionality with a compound comprising a plurality ofallophanate groups to form a resin mixture; (b) salting the ionizablefunctionality and emulsifying the resin mixture in water.